1. Field of the Invention
The present invention relates to a motor casing made of a resin, for use in vehicle-mounted electric equipment or the like, and a method of manufacturing the same.
2. Description of the Related Art
It has been proposed to form a light-weight motor casing (a motor housing) by molding using a crystallizable thermoplastic resin material. However, a motor casing made of thermoplastic resin suffers from a drawback in that the processing accuracy is lower than that of a metal casing. Therefore, in such a resin motor casing, the portion into which a bearing for rotatably supporting a motor shaft is pressingly inserted has an inner peripheral surface which is not round. This allows an excessive amount of localized pressure to act on the bearing, causing localized deformations and reduction in the bearing accuracy.
Furthermore, as shown in FIG. 5, a bearing 6 is inserted into a bearing seat 11a of a bearing seat portion 11 of a casing 10 made of a resin where the bearing seat 11a has an inner diameter smaller than the outer diameter of the bearing 6. With such a casing, the inner diameter of the bearing seat may be increased to such an extent that a required bearing seating pressure cannot be obtained due to the deteriorated processing accuracy. In that case, the bearing may shift out of the bearing seat 11a.
Motors mounted on a vehicle, such as an automobile, are well-known. In a motor casing for use in a vehicle motor, the operational temperature range in which the motor casing is used must be enlarged to include the high temperature end. However, since the coefficient of thermal expansion of the resin material which forms the motor casing is larger than that of the metal which forms the bearing, the inner diameter of the bearing seat 11a may become larger than the outer diameter of the bearing at high operational temperatures. A combination of this expansion, and creeping of the resin material, leads to reduction in the seating pressure on the bearing, thus allowing the bearing to shift from the bearing seat portion.
It has therefore been proposed to reduce the inner diameter of the bearing seat to ensure the required seating pressure at high operational temperatures. However, this makes the seating pressure too large in a normal temperature state, generating galling between the inner peripheral surface of the bearing seat and the outer surface of the bearing, which may lead to planing of the inner peripheral surface of the bearing seat portion or to the cracking of the bearing seat.
In the above-described type of motor casing, a reinforcing material, such as glass fiber, may be mixed in, in order to increase the mechanical strength of the casing. As the reinforcing material has a fiber-like shape and is elongated, an ejected resin material may have a directional property, and have different shrinkage factors in the longitudinal and lateral directions. This increases the tendency for dimensional changes of the motor casing.
Accordingly, attempts have been made to improve the accuracy of the resin casing by maintaining the temperature of the mold used for molding at substantially the recrystallization temperature of the resin material over many hours until the crystallization of the resin material reaches substantially a saturated state, the resin being then cooled. This is effective to restrict changes (shrinkages) in the dimensions caused by the recrystallization of the resin product. However, to obtain a saturated state of crystallization, the temperature must be held constant over many hours. This is unpractical as the most economical manufacturing method is to make casings which are mass produced in a short period of time.
In another conventional manufacturing method, a resin material is injected into a mold which is heated to a lower temperature so as to achieve quick cooling of the resin material and thereby shorten the molding time. However, in this method, amorphous areas may be generated in the resin material due to quick cooling. Such amorphous areas may recrystallize when the casing is used in the vicinity of the glass transition temperature, thus generating changes in the dimensions. Such temperatures are generated when the casing is used for electrical equipment mounted on a vehicle.
Accordingly, it has been proposed to prevent generation of amorphous areas by controlling the speed at which the temperature of the mold is lowered by using a temperature adjusting device mounted on the mold. It has also been proposed to use a mold which is slightly elliptical or eccentric, so that changes in the dimensions caused by molding are taken into consideration and compensated for. Deformation caused by molding may therefore make the casing round. However, in this method, the relation between the temperature lowering speed and the changes in the dimensions of the product and the shape of the mold must be measured using a highly accurate measuring technique. Also, since a very sophisticated and expensive temperature lowering device and an accurate mold processing technique are required, large losses in both time and cost are generated. When the shape of the molded product is complicated, as in the present motor casing, estimation of changes in the dimensions caused by molding is difficult. Therefore, consistent reproducibility of corrected, round, casings is poor.
In another manufacturing method, accuracy of the bearing inserting position is improved by increasing the wall thickness of the bearing so that the seating pressure does not cause deformation of the bearing or by reducing the insertion margin between the bearing and the bearing seat. The insertion margin is the overlap or difference caused by a bearing whose outer diameter is slightly larger than the inner diameter of the bearing seat.
In the former method, the diameter of the motor casing increases by a degree at which the bearing is made thicker, thus increasing the overall size of the motor casing. In the latter method, since the inserting margin is small, the bearing shifts quite easily. Consequently, the bearing supporting strength is reduced.